Evidence of pre-launch characterization problem of Meteosat-7 visible spectral response

Abstract

Since the early 1980s, the geostationary Meteosat instruments have been observing the earth at a high temporal and spatial resolution. The data record contains more than 30 years of observations and has become very useful for climate monitoring. With a compilation of 10 satellites, it is crucial to reach the highest possible consistency between the instruments. However, the quality of the level 1 data records is still far from what is needed to generate homogeneous climate data records, due to both temporal ageing as well as discontinuities between successive instruments. In this letter, a method is proposed to assess the pre-launch Meteosat-7 visible spectral response curve and a model of its ageing. The results not only validate the ageing model, but also provide evidence that the official pre-launch spectral response curve of Meteosat-7 poorly represents the sensitivity of the instrument in the blue part of the visible spectrum. Until the Meteosat-7 visible spectral response curve has been reassessed, it is suggested to replace the Meteosat-7 curve with the curve of the high resolution visible channel of Meteosat-8. Quantitatively, the error introduced when unfiltering the data (i.e. conversion from filtered to unfiltered radiance), due to the uncertainty on the Meteosat-7 spectral response, is estimated at 4.5% using the Meteosat-7 spectral response curve, but could be reduced to 2.1% by following the recommendations in this letter.

Acknowledgements

The authors would like to thank EUMETSAT for providing help with the research. This work was supported by the Climate Monitoring Satellite Application Facility (CM-SAF) of EUMETSAT.

Notes

1. 1. For Meteosat-7 the calibration and offset coefficients are provided by EUMETSAT on their website, for Meteosat-8 they are found in the header of the images.